Stearate anions were successfully intercalated into the layered double hydroxide Mg4Al2(OH)12CO3.3H2O (LDH-CO3) by several methods to form LDH-SA. The intercalation method which involved the acid-base reaction between emulsified stearic acid (SA) and the carbonate anions in aqueous media was studied for the first time. This method led to the formation of more LDH-SA than well known methods such as melting the carboxylic acid in the presence of the LDH, allowing the interlayer region to swell in the presence of glycerol and reconstructing the calcined LDH in the presence of aqueous sodium stearate. Other literature methods involve ion-exchange of Cl- in LDH-Cl with stearate in aqueous sodium stearate, usually under N2 atmosphere. The methods developed in this study are more industrially viable because the more easily produced LDH-CO3 is used and no N2 atmosphere is necessary. The LDH-SA was successfully used to intercalate sodium polyvinyl sulphonate by an ion exchange with the intercalated stearate, without the need for a N2 atmosphere. This method of production could be useful for the production of nanocomposites in general, for example anionic polymer chains (such as DNA) and anionic clays. The same intercalation reaction was allowed to take place in situ during the formation of dextrin-alginate-glycerol film solutions in water-ethanol media. The stearate intercalated as a bilayer in the interlayer region of the LDH. The SA to LDH ratio was varied from 100% SA to 100% LDH. Around the middle of the series a minimum water vapour permeability (WVP) was obtained, which corresponded to an 80% reduction in WVP in comparison to the reference (blank) film. Around the middle of the series a maximum increase in Young’s modulus, corresponding to a 213% increase in comparison to the blank film, was obtained. Around the middle of the series a reduction in the intensity of the basal reflection and interlayer distance showed that some exfoliation (delamination) took place. / Thesis (PhD (Chemistry))--University of Pretoria, 2008. / Chemistry / unrestricted

Além do interesse intrínseco pelos seus aspectos mecanísticos, as reações de hidrólise de compostos carbonílicos apresentam também a interessante particularidade da reação pelo próprio solvente, a água. Dentre estas reações, conhecidas como \"reações de água\" (Robertson, 1967; Johnson, 1967), estudou-se neste trabalho a hidrólise de carbonatos, através do cálculo das energias de transferência da fase gasosa para o solvente, de reagentes (R), estado de transição (ET) e produtos (P). O estudo da solvatação de modelos moleculares para R e ET indica uma correlação entre reatividade e estrutura molecular. Os resultados usando o enfoque de \"super-molécula\" mostram maior concordância com os dados experimentais do que o cálculo de solvatação da molécula simples e indicam que a solvatação dos modelos de ET é mais eficiente que para R e, portanto, há um aumento da reatividade. O estudo mais detalhado das estruturas de R, ET e P, em misturas água/acetonitrila para carbonatos de difenila e bis(2,4-dinitrofenila), sugere a existência de duas ligações de hidrogênio: entre o oxigênio da água do \"cluster\" e um dos hidrogênios dos anéis aromáticos (CF e CDNF), e entre o hidrogênio da água e o oxigênio do grupo nitro do outro anel aromático (CDNF). A consequente diminuição da liberdade conformacional em relação à fase gasosa, provocada por estas ligações de hidrogênio (CF e CDNF), expõe um dos hemisférios da carbonila ao ataque da água, provocando então uma aceleração entrópica do processo. Os efeitos eletrônicos, devidos às ligações de hidrogênio, estão de acordo com a maior acidez esperada dos hidrogênios dos anéis do CDNF em relação ao CF. Também mostram uma compensação no CDNF, pouco contribuindo para alterar a densidade eletrônica no seu carbono carbonílico, enquanto que indica uma soma de efeitos no CF, contribuindo então para um aumento desta densidade eletrônica no CF, de acordo com sua conhecida baixa reatividade. O trabalho permite ainda concluir sobre o relativo sucesso do uso de método semi-empírico PM3 e modelo relativamente simples de solvatação (Cramer & Truhlar, 1991), para o cálculo de energia de transferência em misturas de água/acetonitrila, na faixa de fração molar da água (0,40 a 1,00) onde o método apresenta resultados concordantes com os valores experimentais. / In addition to their intrinsic mechanistic interest, hydrolysis reactions of carbonyl compounds in aqueous media exhibit the interesting peculiarity of direct reaction with the solvent itself, i.e., water. In the present work, we have investigated a representative example of one of these \"water reactions\" (Robertson, 1967; Johnson, 1967), the hydrolysis of carbonates, via quantum chemical ca1culation of the free energies of transfer of the reagents (R), the transition state (TS) and the products (P) from the gas phase to water. A model study of the solvation of R and TS points to a correlation between reactivity and molecular structure. Results using the \"super-molecule\" approach show greater agreement with experiment than solvation ca1culations on the isolated molecule and imply that the solvation of the TS is more effective than that of R in increasing reactivity. A more detailed study of the structures of R, TS and P for diphenyl- (DPC) and bis(2,4-dinitrophenyl)carbonates (DNPC) in acetonitrile/water mixtures suggests the existence of two possible types of hydrogen bonds, i.e., between oxygen of the water c1uster and an aromatic ring hydrogen (DPC and DNPC) or, in the case of DNPC, between the protons of water and the oxygens of the nitro group of the second aromatic ring. The decrease in conformational degrees of freedom reI ative to the gas phase provoked by these hydrogen bonds exposes one of the hemispheres of the carbonyl group to attack by water, resulting in an entropic acceleration of the reaction. The electronic effects on the hydrogen bonds are in line with the greater acidity of the aromatic ring hydrogens of DNPC relative to those of DNP. In DNPC, there is a compensation effect, with very little alteration of the electron density on the carbonyl carbon, while in DPC a sum of effects increases the electron density on the carbonyl carbon, in line with the known lower reactivity of the latter. This work points to the relative success of the semi-empirical PM3 method combined with relatively simple solvation models (Cramer & Truhlar, 1991) for ca1culating free energies of transfer involving acetonitrile/water mixtures in the water mole fraction range from 0.40-1.00.

Despite high temperature and salinity conditions that challenge reef growth in other oceans, the Red Sea maintains amongst the most biodiverse and productive coral reefs worldwide. It is therefore an important region for the exploration of coral reef functioning, and expected to contribute valuable insights towards the understanding of coral reefs in challenging environments. This dissertation assessed the baseline variability of in situ abiotic conditions (temperature, dissolved oxygen, pH, and total alkalinity, among others) in the central Red Sea and highlights these environmental regimes in a global context. Further, focus was directed on biotic factors (biofilm community dynamics, calcification and bioerosion), which underlie reef growth processes and are crucial for maintaining coral reef functioning and ecosystem services. Using full-year data from an environmental cross-shelf gradient, the dynamic interplay of abiotic and biotic factors was investigated. In situ observations demonstrate that central Red Sea coral reefs were highly variable on spatial, seasonal, and diel scales, and exhibited comparably high temperature, high salinity, and low dissolved oxygen levels, which on the one hand reflect future ocean predictions. Under these conditions epilithic bacterial and algal assemblages were mainly driven by variables (i.e., temperature, salinity, dissolved oxygen) which are predicted to change strongly in the progression of global climate change, implying an influential bottom up effect on reef-building communities. On the other hand, measured alkalinity and other carbonate chemistry value were close to the estimates of preindustrial global ocean surface water and thus in favor of reef growth processes. Despite this beneficial carbonate chemistry, calcification and carbonate budgets in the reefs were not higher than in other coral reef regions. In this regard, seasonal calcification patterns suggest that summer temperatures may be exceeding the optima of calcifiers. As a possible interpretation of the here observed environmental regimes, it can be concluded that the central Red Sea may be less sensitive to ocean acidification, but is already impacted by ocean warming. Importantly, this dissertation provides valuable present-day baseline data of the natural variability of relevant abiotic drivers together with benthic community and reef growth dynamics. These data will be important for future comparative studies and efforts to quantify the impact of future environmental change in the region.

During the past decades, emission of greenhouse gases has accelerated to unsustainable levels. This is a serious issue that can have a devastating impact on everything from global economy to the terrestrial or marine ecosystem. A method for reducing the emission is named carbon capture and storage, which this project is based on. In this study, different concentrations of citric acid (CA) is used (as an additive) for the enhancement and optimization of carbon dioxid sorption properties of amorphous calcium carbonate (ACC), mesoporous magnesium carbonate (MMC) and calcium magnesium carbonate composite (CMC). These materials were heat treated in a calcination and an alternating carbonation process in order to study the carbon dioxid sorption performance. During the calcination process, CA undergoes a pyrolysis reaction in order to increase the specific surface area of the individual nanoparticles, which is an important factor for the sorption capacity. In the case of CMC, different molar ratios of magnesium oxide and calcium oxide were used in order to alter the concentration of the resulting magnesium oxide prior to heating. All three materials consisted of aggregations of nanometer-sized particles. Thermogravimetric analysis, scanning electron microscopy, surface area and porosimetry and infrared spectroscopy analysis suggest that the carbon dioxid sorption properties and the sintering stability of ACC and MMC do not improve since CA evaporates due to pyrolysis. Sintering was a greater problem for the evaluated CA treated ACC sample. However, in the case of CMC, the sorption and sintering properties were enhanced due to the higher Tamman-temperature of magnesium oxide, specifically for the lower concentration of magnesium oxide. After 19 carbonation cycles, CMC-1:1-25% CA showed signs of improved sintering stability and sorption capacity, compared to ACC-75% CA. / <p>Presentationen genomfördes på distans.</p>

Thallium (Tl) is a rare heavy metal in drinking water, but its extreme toxicity makes its removal crucial to consumer health. Traditional treatment methods do not work for Tl in sources with high concentrations of ions like calcium (Ca+2) and potassium (K+), as they are removed preferentially to Tl. A treatment method that specifically targets Tl must be applied. Pilot studies conducted in Park City Utah found that pyrolusite, a manganese oxide ore, will remove Tl to very low concentrations in the presence of competing ions but because this method is not common, further study was required. This study investigated the mineral composition of the pyrolusite and where, within the material, Tl was held. Calcite is known to accumulate Tl so tests were done looking at Tl accumulation onto pyrolusite with and without calcite on the surface in three water qualities: one with high pH and chlorine (Cl2) and low concentrations of the interfering ions Ca, K, chloride (Cl-) predicted to promote Tl removal, one with low pH and Cl2 and high concentrations of interfering ions predicted to limit Tl removal, and one with a moderate pH used for comparison. The likelihood of this Tl to stay on the media surface with changes in water chemistry was also tested. It was seen that Tl does accumulate in calcite on the media surface. Both pyrolusite media showed a high capacity for Tl and media without calcite exhibited a preference for Tl over K. Calcite dissolved with changes in pH which led to the conclusion that its formation on the media surface must be closely monitored as it may increase the risk Tl release into the drinking water system.

Holocene carbonate sediments from Mexican reefs in the Caribbean and Gulf of Mexico display variations in constituent composition, texture and mineralogy which are related to their locations on the reefs. Samples were collected at barrier reef environments at Akumal and Chemuyil, on the northeast coast of the Yucatan Peninsula; at the oceanic atoll of Chinchorro, off the southeast coast of the peninsula; and at the shelf atoll of Alacranes, in the Gulf of Mexico. Samples were collected through a depth range of 0-40 m, which encompasses back reef, shallow fore reef, and deeper fore reef environments. Constituent-particle analysis of reef sediments indicates that lagoon facies are dominated by Halimeda with lesser amounts of coral and coralline algae. In contrast, fore reef facies are dominated by coral, with lesser amounts of Halimeda and coralline algae. Greater than 90% of the sediments (dry weight) occur in the interval 0.125-2.00 mm, with mean grain sizes (Mz) approaching 0.5 mm at most sites. Mz generally decreases with increasing depth at three of the four sites. Reef sediments are moderately to poorly sorted and typically become more poorly sorted with increasing depth. Sediments collected from reef channels (grooves) are consistently better sorted than those from the reef interstices (spurs). Mineralogically, the sediment is predominantly aragonite (63-93%) and high-Mg calcite (3-33%), with minor amounts of low-Mg calcite (3-9%). The sediment is chiefly composed of these three polymorphs of CaC03 (96.32- 99.83%) with only a small percentage of insoluble (non-carbonate) material. The non-carbonate fraction of the sediment is dominated by organics (0.14-3.16%) with lesser amounts of clay minerals and amorphous silica (0.00-0.66%). Quantitative analysis of Mexican reef sediments in the Caribbean and Gulf of Mexico allows the delineation of reef zonations based on constituent-particle composition. Q-mode cluster analysis of constituent-particle data from epireefal sediments enables the separation of lagoon, shallow fore reef (10-15 m) lithofacies. These groupings were maintained even when constituent-particle data from the barrier reefs were combined, and also when data from the atoll reefs were combined. Similar cluster groupings were obtained when constituent-particle data from a Jamaican fringing reef were combined with data from the Mexican barrier reefs. This indicates that coastal reef sediments from Jamaica and Mexico are surprisingly similar in constituent-particle composition and facies zonations. However, cluster analysis failed to produce clear associations when constituent-particle data from barrier reefs and atoll reefs were combined. The sedimentological zonations observed in the study reflect the community composition of the living reefs, and indicate that community composition, at least for calcifying organisms, is potentially preservable.

Evolution of rocky coastlines is controlled by littoral, biological and fluvial processes. Resultant landforms are overprinted and/or new ones formed as a result of changes in sea level caused by glacioeustasy and/or local tectonics. On carbonate coasts, chemical erosion in the form of karstification takes on a dominant role. Type of karstification is an important factor in understanding carbonate coast evolution and landform development so it is critical to identify type of karstification. In this research, fractal indices were used to distinguish cave and thus karstification type. It was determined that fractal indices effectively differentiated cave types and the indices were used to distinguish cave types at study sites on Barbados, the ABC Islands (Aruba, Bonaire, Curaçao) and the Caribbean coast of the northeast Yucatan peninsula, Mexico. This research evaluated caves located in the phreatic, epiphreatic and vadose zones of the northeast coast of Quintana Roo, Mexico to determine the relationship between the caves and to coastal processes. Three distinct coastal landforms associated with caves on the study sites were evaluated to quantify and model the interplay of littoral, fluvial and karstic processes and cave and karst development. On Barbados, the combination of surface fluvial processes, and mixing-zone and fluvial-karstic dissolution, resulted in the formation of gullies. Some gullies contained caves in their bounding walls and/or served as points of recharge to fluvial caves. Bokas of the ABC islands are distinctive geomorphic structures that formed from the interplay of fluvial, littoral and mixing zone karstification. The morphology of the bokas was a function of dominant geomorphic process. The caletas of the Yucatan Caribbean were formed by karstification processes that also produced features with mixing-zone-like morphologies but with fluvio-karstic function. The results of this research expand the Carbonate Island Karst Model (CIKM), which explains eogenetic dissolutional processes and landforms on small carbonate islands, to one that includes carbonate islands of all sizes, and carbonate continental coasts.

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